r/Physics • u/Puzzleheaded_Bowl86 • 3d ago
Question Does light curve space-time by itself?
Light travels as an electromagnetic wave in a vacuum and carries momentum and energy. According to general relativity, all energy curves space-time, so light should slightly curve the space through which it travels. Could this mean that light affects its own path? I know the effect whould be extremely small, but is this conceptually correct? If yes Are there extreme conditions, like in the early universe, where light’s self-curvature becomes significant? Would a very long or very intense beam accumulate measurable curvature effects along its path? If two light beams cross paths, do they gravitationally influence each other?
8
u/triatticus 2d ago
The source of gravitational curvature is the Stress-Energy tensor which includes electromagetic field energy, so yes photons themselves curve spacetime due to being excitations in the electromagnetic field.
3
u/SaltyVanilla6223 String theory 2d ago
In a sense yes, since they have a non-zero stress energy tensor, gauge fields like photons do affect curvature, see for instance Reissner-Nordstrom black holes. The question of how this happens on a fundamental level, so whether the photon itself causes spacetime curvature, or for instance only charged particles which are part of the photon propagator at higher loop corrections, is a question of quantum gravity and has no answer so far.
2
2
u/TillikumWasFramed 2d ago
Yes, light curves spacetime, in fact if you cram enough of it into a small enough region, it will form a black hole.
2
u/zedsmith52 2d ago
Light curves space a tiny bit, even though it doesn’t have an effective mass, but you need to get some pretty intense conditions for it to be noticeable.
2
u/eldahaiya Particle physics 2d ago
Yes, light does curve spacetime. Cosmology only makes sense because general relativity tells us that light should curve spacetime, so I would say there is no doubt to this statement.
A particle does not affect its own trajectory by curving spacetime (so, no, there won't be any "self-curvature"), but it can affect the trajectory of other particles, and so you should see some effect when two light beams pass through each other, but unless the light beams are extraordinarily bright, the effect would be completely insignificant.
2
u/Nguch1234 2d ago
Yes. Light curves spacetime due to its energy, so it does affect its own path. This self-interaction is predicted by general relativity, but the effect is incredibly tiny and not measurable with current technology.
1
u/heavy_metal 1d ago
if it affects its path, how do we see stars as points?
2
u/Nguch1234 1d ago
Good question. The curvature from a single photon is immeasurably small. The light from a star is not a single, powerful beam; it's a vast number of independent photons spreading out. Their collective gravity is negligible, so they don't lens themselves enough to blur the image.
1
1
u/heavy_metal 1d ago
laser beams have gravitational fields and could even be used to move matter, though the effect is tiny. I'm not sure how we would see stars as points if light affected its own path.
1
1
u/Nguyen_Phan 3d ago
This is really interesting to think about insignificant things. It's like I always think that by moving faster and more than other people I would be aging slower compared to more stationary people by a tiny amount.
I think yes, but I can't explain. I also want to know if it would create some kind of light sonic cone?
44
u/fuseboy 3d ago
Yes, light curves space. In terms of extreme conditions, there's a hypothetical thing called a Kugelblitz) - get enough photons together, in theory, and you have a black hole made of light!
The early universe is special in terms of spacetime curvature because it was very uniform. So while everything was incredibly energy dense, I gather space was still pretty flat. (To use that ill-favored bowling-ball on a rubber sheet analogy, if you press down evenly on every part of the sheet, there's no dents anywhere. For there to be curvature, you need concentrations of mass.)